In recent years, a wireless terminal such as a mobile-phone, a wireless LAN (Local Area Network) or the like has remarkably become widespread. A frequency band to be used also extends to a GHz band. Therefore, the use of new frequency bands in communications falls into a difficult situation.
Under such conditions, a communication system using impulse-like pulse trains whose pulse widths are extremely narrow (in the neighborhood of 1 ns, for example) has become a focus of attention as a new use method. As such a communication system using the pulse trains, there is known, for example, an ultra wide band, impulse radio (hereinafter called “UWB-IR” for short) communication system. As its one example, a UWB-IR communication system for modulating Gaussian mono pulses by a pulse position modulation (PPM) system has been disclosed in a non-patent document 1.
In these communication systems using the pulse trains, the transmission of information is done by sending and receiving of an intermittent energy signal as distinct from signal transmission using a normal continuous wave.
Since each of the pulses that constitute each pulse train has the pulse width very narrow as described above, its signal spectrum spreads in frequency band as compared with the communication using the normal continuous wave, and the energy of the signal is dispersed. As a result, the signal energy per unit frequency band becomes small. Thus, communications are enabled without causing interference with other communication system, and the sharing of the frequency band is enabled.
As one application field of the UWB-IR communications, may be mentioned, a wireless sensor network (hereinafter called “sensor network” for short) technique. The sensor network transmits the conditions of persons and things, various conditions such as ambient surroundings for them and environmental data onto a network to realize an increase in the efficiency of business. Further, a novel application has been expected as well. Particular attention is given to the utilization of security, healthcare and the like. Since the UWB-IR communication system is low in power consumption and small in size, it has been expected as a communication apparatus for the sensor network which makes use of terminals each provided with wireless equipment in large quantities.
As this type of related art example, a demodulator capable of controlling gain relative to an analog modulation signal of amplifying means on the basis of amplitude information of a baseband signal obtained as quadrature detection has been disclosed in the patent document 1.
There is also known a technique for controlling signal capture and transition of tracking by a controller in an ultra wide band communication system (refer to, for example, a patent document 2).
There is further known an AGC circuit wherein upon setting the gain of amplifying means by use of a spectrum-spread received signal, gain control is performed using a received signal (on which noise is being superimposed) prior to being inputted to a correlator before synchronous retention, and gain control is carried out using a received signal (from which noise is being removed) spectrum inversely-spread by the correlator after synchronous establishment is made (refer to, for example, a patent document 3).
[Patent document 1]
    Japanese Patent Laid-Open No. H9 (1997)-238171[Patent document 2]    International Patent Publication No. 02/32066, pamphlet (see FIG. 1a)[Patent document 3]    Japanese Patent Laid-Open No. 2003-218722[Non-patent document 1]    Moe Z. Win et al, “Impulse Radio: How it works”, US Patent document, IEEE Communications Letters, Vol. 2, No. 2, p 36-38 (February, 1998)